A synthetic hydrogel, a natural hydrogel, or a polymer is a soft material hardened from a liquid state having viscosity into a solid state.
Particularly, a hydrogel among soft materials means a porous material formed of a water-soluble polymer material. The porous material, which is a material having a porous structure, has characteristics that properties such as mechanical strength, permeability, electro-conductivity, and the like, are changed depending on factors such as porosity, sizes of pores, a distribution of the pores, shapes of the pores, and the like. The porous material has been variously used as materials of filters, electrodes, gas sensors, gas separators, scaffolds in the case of transplanting a biological tissue, wet dressing, mask packs, and the like, using these characteristics.
In the hydrogel, which is a kind of polymer, monomers and cross-linkers are combined with each other to form pore structures capable of holding water therein. Since the hydrogel has excellent hydrophilicity and biocompatibility, many studies on the hydrogel for medical applications have been conducted. For example, since the hydrogel holds water therein, the hydrogel may provide an advantageous environment to cells, drugs, or the like, is appropriate for carrying nutrients required for cultivation of cells, is easily deformed by a cell adhesion ligand, and does not have biological toxicity, such that the hydrogel is used as a material of a tissue engineering scaffold, a drug delivery system, a patch for wound healing, or the like, and recently, studies on applications of a structure of a soft robotics and a microfluidic actuator have been actively conducted.
However, the hydrogel has a disadvantage such as weak mechanical strength, such that there is a limitation in using the hydrogel in an actual application. Therefore, many studies for improving a mechanical property, or the like, of the hydrogel by applying a specific stimulus to the hydrogel have been conducted. Particularly, the hydrogel has been used to culture stem cells, and it has been known that a stiffness of the hydrogel has an influence on differentiation of the stem cells. That is, even though stem cells are the same each other, cells formed by differentiating the stem cells become different from each other depending on a stiffness of the hydrogel used to culture the stem cells (see FIG. 1). Therefore, it acts as a very important element in determining an application range of the hydrogel to control a mechanical strength of the hydrogel.
As an attempt to control a mechanical property of the hydrogel, there is a method of improving ductility and toughness in a specific direction using a double-network hydrogel. However, in a process of manufacturing the double-network hydrogel, several complicated steps should be performed, and there is a limitation in a material that may be used to form a primary structure network. As another method, there is a directional freezing-thawing method. However, this method has a limitation in a forming temperature, and the possibility that damage will be generated in a material in a freezing-thawing process is high, such that this method is not appropriate for being used for a biological material. Recently, a study for controlling a mechanical property of the hydrogel by adding an ultrasonic wave to the hydrogel has been conducted (see FIG. 2). However, this study does not also provide a method capable of controlling stiffnesses of each region in the hydrogel, such that it is not appropriate for being used to manufacture the hydrogel.